Transdermal drug delivery systems may be designed to act locally at the point of application or to act systemically by entering the body's blood circulation. In these systems, delivery may be achieved by direct topical application of a substance or drug in the form of an ointment or the like, or by adhesion of a patch with a reservoir that holds the drug and releases it to the skin in a time-controlled fashion.
Transdermal delivery systems for agents such as drugs, pain relieving compounds, vitamins, and skin improving compounds have been in use for a number of years. However, these systems have typically only been useful for transdermal delivery of relatively small molecules. The skin's porous structure permits such small molecules to pass from the epidermis to the dermis via diffusion. These transdermal delivery systems such as creams have been developed for use with analgesics and skin refining compounds. Transdermal systems using a patch have been developed for nicotine and estrogen therapies. Estradiol technologies are described in U.S. Pat. No. 6,521,250 to Meconi, et al. However, large molecules, such as insulin, are not able to diffuse through the skin. To date there has not been an effective and economical method to transport such molecules through the epidermis to enter the bloodstream via the dermal vasculature.
It has been proposed that molecules, potentially including larger molecules, can be transported through the skin when such molecules are contained within spherical vesicles, variously called microparticles, microspheres, liposomes, lipid vesicles, transfersomes, formed by hydrating a phospholipid. The resulting vessels are water-insoluble and are dispersed and suspended in a liquid base material which is applied to the skin to deliver the drug. U.S. Pat. No. 6,165,500 to Cevc discloses “transfersomes,” which are vesicles containing both a lipid and surfactant, to achieve transdermal delivery, based on a theory that osmotic pressure will drive the transfersomes through the dermis. Other solutions have been proposed, including the use of ultrasound devices to generating shock waves to enlarge pores, use of electric current to drive substances across skin, and the use of microneedles to pierce skin and deliver drugs into bloodstream. (See More Than the Patch: New Ways to Take Medicine Via Skin, New York Times, Jul. 2, 2002, page F5.)
There remains a need for a transdermal drug delivery system with the improved skin permeability and ability to transport a wider range of substances or drugs. This problem is particularly apparent in the transdermal delivery of substances composed of large molecules, such as polypeptides or proteins, which do not readily pass through the pores of the skin. Absent such a transdermal drug delivery system, the use of injections to deliver these substances will remain the conventional dosage method, despite the pain, complicated administration and general invasiveness involved therein.